Semiconductors are used in integrated circuits for electronic applications, including radios, televisions, cell phones, and personal computing devices, as examples. One type of semiconductor device is a semiconductor storage device, such as a dynamic random access memory (DRAM) and flash memory, which use a charge to store information.
A more recent development in memory devices involves spin electronics, which combines semiconductor technology and magnetics. The spin of an electron, rather than the charge, is used to indicate the presence of a “1” or “0”. One such spin electronic device is a magnetic random-access memory (MRAM), which includes conductive lines positioned perpendicular to one another in different metal layers, the conductive lines sandwiching a magnetic stack. The place where the conductive lines intersect is called a cross-point. A current flowing through one of the conductive lines generates a magnetic field around the conductive line and orients the magnetic polarity into a certain direction along the wire or conductive line. A current flowing through the other conductive line induces the magnetic field and can partially turn the magnetic polarity, also. Digital information, represented as a “0” or “1”, is storable in the alignment of magnetic moments. The resistance of the magnetic component depends on the moment's alignment. The stored state is read from the element by detecting the component's resistive state. A memory cell may be constructed by placing the conductive lines and cross-points in a matrix structure having rows and columns. Magnetic stacks or cells in the cross-point array are usually selected by passing sub-threshold currents through the conductive lines, e.g., in both the x- and y-direction, and where the conductive lines cross, the combined magnetic field is large enough to flip the magnetic orientation.
An advantage of MRAMs compared to traditional semiconductor memory devices such as DRAMs is that MRAMs are non-volatile. For example, a personal computer (PC) utilizing MRAMs would not have a long “boot-up” time as with conventional PCs that utilize DRAMs. Also, an MRAM does not need to be powered up and has the capability of “remembering” the stored data.
Alignment techniques are implemented during manufacturing processes to ensure correct alignment of the various layers with one another within semiconductor devices such as MRAMs. Typically, alignment marks are utilized in the layers to assist in the alignment of features in different layers.
MRAM devices are typically processed using structures upon which are formed a plurality of magnetic metal stacks which comprise the magnetic memory cells. A magnetic stack comprises many different layers of metals and a thin layer of dielectric having a total thickness of a few tens of nanometers. The magnetic stacks are typically built on top of copper wires embedded in an inter-level dielectric (ILD) material.
Because the magnetic stacks are not transparent to light, the lithography on top of the magnetic stack layer requires topographic features for alignment and overlay measurement marks on the magnetic stack layer. Typically, this underlying magnetic stack layer requires a chemical mechanical polish (CMP) process as a finish step.
Alignment marks are usually formed using additional lithography and reactive ion etch (RIE) steps to generate marks on the CMP-finished surface that exposes the copper and dielectric patterns. However, forming alignment marks in this manner requires an additional RIE process step and subsequent cleaning steps, thus increasing the processing costs and also increasing the chance of leaving particles on the CMP finished level. Also, the additional lithography mask must be aligned to an underlying layer, which reduces the overall overlay tolerance.